[clang] Respect field alignment in layout compatibility of structs (#84313)

This patch implements
[CWG2586](https://cplusplus.github.io/CWG/issues/2583.html) "Common
initial sequence should consider over-alignment". Note that alignment of
union members doesn't have to match, as layout compatibility of unions
is not defined in terms of common initial sequence
(http://eel.is/c++draft/class.mem.general#25).

1 files changed, 21 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 3597f93..b34b8df 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -19184,8 +19184,22 @@ static bool isLayoutCompatible(ASTContext &C, EnumDecl *ED1, EnumDecl *ED2) {
 }
 
 /// Check if two fields are layout-compatible.
+/// Can be used on union members, which are exempt from alignment requirement
+/// of common initial sequence.
 static bool isLayoutCompatible(ASTContext &C, FieldDecl *Field1,
-                               FieldDecl *Field2) {
+                               FieldDecl *Field2,
+                               bool AreUnionMembers = false) {
+  const Type *Field1Parent = Field1->getParent()->getTypeForDecl();
+  const Type *Field2Parent = Field2->getParent()->getTypeForDecl();
+  assert(((Field1Parent->isStructureOrClassType() &&
+           Field2Parent->isStructureOrClassType()) ||
+          (Field1Parent->isUnionType() && Field2Parent->isUnionType())) &&
+         "Can't evaluate layout compatibility between a struct field and a "
+         "union field.");
+  assert(((!AreUnionMembers && Field1Parent->isStructureOrClassType()) ||
+          (AreUnionMembers && Field1Parent->isUnionType())) &&
+         "AreUnionMembers should be 'true' for union fields (only).");
+
   if (!isLayoutCompatible(C, Field1->getType(), Field2->getType()))
     return false;
 
@@ -19204,6 +19218,11 @@ static bool isLayoutCompatible(ASTContext &C, FieldDecl *Field1,
   if (Field1->hasAttr<clang::NoUniqueAddressAttr>() ||
       Field2->hasAttr<clang::NoUniqueAddressAttr>())
     return false;
+
+  if (!AreUnionMembers &&
+      Field1->getMaxAlignment() != Field2->getMaxAlignment())
+    return false;
+
   return true;
 }
 
@@ -19265,7 +19284,7 @@ static bool isLayoutCompatibleUnion(ASTContext &C, RecordDecl *RD1,
         E = UnmatchedFields.end();
 
     for ( ; I != E; ++I) {
-      if (isLayoutCompatible(C, Field1, *I)) {
+      if (isLayoutCompatible(C, Field1, *I, /*IsUnionMember=*/true)) {
         bool Result = UnmatchedFields.erase(*I);
         (void) Result;
         assert(Result);